The presents invention relates to internal combustion engines of the type comprising:
at least one intake valve and at least one exhaust valve per cylinder, each equipped with respective return spring devices that bring the valve back to the closed position in order to control communication between their corresponding intake and exhaust manifolds and the combustion chamber;
one camshaft to drive the intake and exhaust valves of the engine cylinders by means of appropriate tappets;
in which at least one of said tappets drives its respective intake or exhaust valve by effect of the movement of the above return spring devices through the operation of hydraulic devices, which include a pressurized fluid chamber.
Said pressurized fluid chamber can be connected to an outlet, by means of a solenoid valve, so as to disconnect the valve from its respective tappet, thus causing the valve to close rapidly by effect of the movement of the corresponding return spring devices.
Said hydraulic devices also comprise a piston joined to the valve stem and mounted so as to slide inside a guide bushing; said piston opens onto a variable volume chamber, which is formed by the piston inside the guide bushing; said variable volume chamber communicates with the pressurized fluid chamber through an opening at the end of the guide bushing; said piston has an end piece that is suitable for insertion inside said opening at the end of the guide bushing, at the end of the closing stroke of the valve, in order to reduce the opening for communication between said variable volume chamber and said pressurized fluid chamber, thus checking the valve""s stroke close to its closing point.
The type of engine specified above is described and illustrated in the applications for European patent, EP-A-0 803 642 and EP-A-1 091 097 filed by the same Requesting Company.
For the above type of engines, it has already been proposed that two valves of the same engine cylinder, e.g. two intake valves or two exhaust valves, be driven by the same cam of the engine distribution shaft.
This problem is usually solved mechanically by mounting a small plate that works in unison with the driving cam, which is, in turn, functionally connected to two tappets that are joined to the valves to be controlled. This solution creates problems in the structure of the engine, also resulting in larger (engine) dimensions and higher costs. In theory, it is possible to provide for a single tappet, connected to the above pressure chamber, and then hydraulically connect the pressure chamber to the chambers that are connected to the driving pistons of the two valves. However, this is not the best solution because it entails a considerable increase in the volume of the hydraulic circuit for driving the valves, to the detriment of the system""s elasticity and making it impossible to work at high engine r.p.m.
The purpose of the present invention is to solve the above problem in a satisfactory manner by applying simple, economic means, which would make it possible to simultaneously control two valves of the same engine cylinder through the same distribution shaft cam.
With a view to achieving this objective, the subject of this invention is an engine with all the characteristics specified in the introduction, which is also characterized in that the above-cited tappet comprises two separate piston sections that are connected to two distinct pressure chambers; these chambers control the driving pistons of two valves of the same engine cylinder.
In one of its preferred arrangements, the above tappet has a cylindrical body with a first piston section of larger diameter and a second piston section of smaller diameter mounted so that they slide inside the corresponding sections of a cylindrical guide bushing; said piston section of smaller diameter opens onto a pressure chamber, which is located at its end. The second pressure chamber consists of a ring-shaped cavity formed inside the portion of the guide bushing of larger diameter by the section of the body of the tappet of smaller diameter. The above two pressure chambers are connected by separate manifolds to the two driving pistons of the valves to be controlled.
Thus, thanks to the characteristics described above, the invention ensures that two valves of the same engine cylinder are driven, starting from a same cam of the distribution shaft, without entailing any problems of construction nor requiring any substantial increase in the volume of the hydraulic circuit for driving the valves. This makes it possible to implement a system with comparably high rigidity, which is therefore capable of responding appropriately even when the engine operates at high r.p.m.